1. Field
Embodiments of the invention relate to an apparatus, system, and method for nanostructure-based devices for electrical energy management. More particularly, embodiments of the invention relate to an apparatus, system, and method for providing nanostructure arrays as two-terminal devices, for example, nanotube or nanowire devices (hereinafter referred to as “nanodevices”), configured to store electrical energy, to capture and generate energy, and to integrate with power management circuitry.
2. Description of the Related Art
The limitations of conventional capacitor and battery devices are well known. Charge storage devices can exhibit similar limitations experienced by conventional solar cell devices, as will be discussed below. Electrostatic capacitors that store charge at the surface of electrodes typically do not achieve high areal densities of the electrodes. Electrochemical supercapacitors and batteries that store charge inside their active surfaces and at the surface also can experience similar limitations experienced by conventional solar cell devices, as will be discussed below. While sub-surface charge storage can enhance energy density, the resulting slow ion/charge transport into these materials can limit available power.
The limitations of conventional devices for energy capture and storage are also well known. In semiconductor pn junction solar cells, planar device layers typically create only a single depletion layer over the surface to separate photo-induced carriers. As a result, each substrate of a semiconductor pn junction solar cell can be limited to only a single active layer. Furthermore, some of the semiconductor material can absorb light, producing excitations outside a depletion range. This can prevent the separation of positive and negative charges and the collection of harvested light energy.
Currently, alternate solar cell structures are being explored that utilize nanocomposite structures that mix nanoparticles, such as C-60 or carbon nanotubes, with organic materials having random spatial distributions on a nanoscale. These nanocomposite structures can provide a high density of interfaces between the component materials, effectively enhancing the active regions, analogous to depletion regions in pn junction semiconductor structures, where charge separation can occur.
However, the nanoscale randomness of the component materials can impede efficient collection of charges at micro- or macro-scale external contacts where charge should be produced, for example, through high electrical resistance through which the charge reaches the contacts. Additionally, these materials, such as conducting polymers, can have relatively high resistivity, further diminishing the efficiency of charge collection at the external contacts.
A number of nanostructures have been explored to improve the power and energy density of conventional capacitor and battery devices and conventional solar cell devices, primarily exploiting higher surface area densities per unit volume of material used in these devices. For example, a high density of nanowires on a surface can substantially enhance the surface area available, producing higher charge density per unit planar area. Furthermore, nanowire and nanotube structures can present shortened pathways for ion transport into the surface, thereby increasing power density. These advancements in technology promise improvements in energy devices, particularly if nanostructures can be formed with sufficient control at the nanoscale to realize functioning and reliable aggregation of massive arrays of nanostructures into larger working devices addressed at the macro- or micro-scale external contacts.
Nanotechnology provides new options for meeting these requirements, particularly using self-assembly phenomena and self-alignment to build more complex nanodevices from simpler nanostructures. For example, anodic aluminum oxide (AAO) can achieve highly regular arrays of nanopores through specific recipes for anodic oxidation of aluminum. Nanopores in AAO may have uniform size and spacing in a hexagonal pattern.